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Unit of power

Physicists use everyday words in seemingly strange and very specific ways. For a physicist, work W isn't what you do from nine to five on weekdays. It's the product of the force F applied to an object multiplied by the distance d the object moves as a result of that force.

Defining Power in Physics

If the object doesn't move, no work has been done. Try explaining that to the man who is trying to push your car out of a ditch and having no luck getting the car to move. Physicists also use the word power P in a specific way. For them power isn't what you get by eating a hearty breakfast.

It's the time t it takes to do a certain amount of work. In other words, power is the rate of doing work. It's also the rate of transfer of heat and electrical energy. Knowing what the word "power" means to physicists helps you understand the units of power. In the SI metric system, the units are watts. All the other units are derived from these. In the SI system, length is measured in meters, mass in kilograms and time in seconds. The joule is also the unit of energy in the SI system, although it's customary to use ergs when measuring in centimeters and grams.

Scientists have another name for this unit, too. It's the watt, named after James Watt, the Scottish inventor who helped develop the steam engine that powered the Industrial Revolution. In the SI system, the watt is defined as:. If you like cars, you know the power ratings of car engines are always given in horsepower. That means horsepower is also a unit of power, but where did it come from, and why is it still in use? It turns out that none other than James Watt is the man responsible for this unit of power.

In order to sell his steam engines, he had to quantify the amount of work they could do in a certain time. He created a unit based on how much work a single pit pony could do. It was well known at the time that a single pony could lift pounds of coal up a foot mine shaft in one minute.

In SI units, that's As a unit of power, the horsepower is usually reserved for engines and — sometimes — air conditioner cooling capacity. Why do we still use it? Probably for the same reason that people in some countries, including the United States, still use the Imperial system of measurement: habit.

Chris Deziel holds a Bachelor's degree in physics and a Master's degree in Humanities, He has taught science, math and English at the university level, both in his native Canada and in Japan.

He began writing online inoffering information in scientific, cultural and practical topics. His writing covers science, math and home improvement and design, as well as religion and the oriental healing arts. If you're measuring in the centimeters and grams, use this conversion:. About the Author. Photo Credits. Copyright Leaf Group Ltd.The watt symbol: W is a unit of power.

In the International System of Units SI it is defined as a derived unit of 1 joule per second[1] and is used to quantify the rate of energy transfer. When an object's velocity is held constant at one meter per second against a constant opposing force of one newtonthe rate at which work is done is one watt.

In terms of electromagnetismone watt is the rate at which electrical work is performed when a current of one ampere A flows across an electrical potential difference of one volt Vmeaning the watt is equivalent to the volt-ampere the latter unit, however, is used for a different quantity from the real power of an electrical circuit.

Two additional unit conversions for watt can be found using the above equation and Ohm's law. The watt is named after the Scottish inventor James Watt. In Octoberat the International Conference on Electric Units and Standards in London, [8] so-called "international" definitions were established for practical electrical units.

After the 9th General Conference on Weights and Measures inthe "international" watt was redefined from practical units to absolute units i. Texts written before are likely to be using the "international" watt, which implies caution when comparing numerical values from this period with the post watt.

Technologically important powers that are measured in femtowatts are typically found in references to radio and radar receivers. For example, meaningful FM tuner performance figures for sensitivity, quieting and signal-to-noise require that the RF energy applied to the antenna input be specified.

What Is the Unit of Power?

These input levels are often stated in dBf decibels referenced to 1 femtowatt. This is 0. Technologically important powers that are measured in picowatts are typically used in reference to radio and radar receivers, acoustics and in the science of radio astronomy.

One picowatt is the international standard reference value of sound power when this quantity is expressed as a level in decibels.

Important powers that are measured in nanowatts are also typically used in reference to radio and radar receivers. Important powers that are measured in microwatts are typically stated in medical instrumentation systems such as the EEG and the ECGin a wide variety of scientific and engineering instruments and also in reference to radio and radar receivers. Compact solar cells for devices such as calculators and watches are typically measured in microwatts.

A typical laser pointer outputs about five milliwatts of light power, whereas a typical hearing aid for people uses less than one milliwatt. The kilowatt kW is equal to one thousand 10 3 watts. This unit is typically used to express the output power of engines and the power of electric motors, tools, machines, and heaters. It is also a common unit used to express the electromagnetic power output of broadcast radio and television transmitters.

One kilowatt is approximately equal to 1. A small electric heater with one heating element can use 1. The average electric power consumption of a household in the United States is about one kilowatt.

A surface area of one square meter on Earth receives typically about one kilowatt of sunlight from the sun the solar irradiance on a clear day at mid day, close to the equator.

Power (physics)

The megawatt MW is equal to one million 10 6 watts. Many events or machines produce or sustain the conversion of energy on this scale, including large electric motors; large warships such as aircraft carriers, cruisers, and submarines; large server farms or data centers ; and some scientific research equipment, such as supercollidersand the output pulses of very large lasers. A large residential or commercial building may use several megawatts in electric power and heat.

The OED also states that megawatt appeared in a 28 November article in the journal Science This unit is often used for large power plants or power grids. For example, by the end of power shortages in China's Shanxi province were expected to increase to 5—6 GW [18] and the installed capacity of wind power in Germany was The terawatt TW is equal to one trillion 10 12 watts. The total power used by humans worldwide is commonly measured in terawatts.

The most powerful lasers from the mids to the mids produced power in terawatts, but only for nanosecond time frames. The average lightning strike peaks at 1 terawatt, but these strikes only last for 30 microseconds. One such laser is the Lawrence Livermore's Nova laserwhich achieved a power output of 1. The duration of the pulse was roughly 0.

Based on the average total solar irradiance [25] of 1. In the electric power industrymegawatt electrical MWe [26] or MW e [27] refers by convention to the electric power produced by a generator, while megawatt thermal or thermal megawatt [28] MWt, MW tor MWth, MW th refers to thermal power produced by the plant.Power is increased if work is done faster or energy is transferred in less time.

If work is done faster, power is higher. If work is done slower, power is smaller. More power is seen when the system is both strong in force and fast in velocity. Power is measured in energy joules divided by time. Power is a scalar quantity, it has no direction. Horsepower is often used to describe the power delivered by a machine.

Horsepower is a unit of power in the British system of measurement. It is the power required to lift pounds by one foot in one second and is about watts. The watt is often seen in relation to light bulbs.

In this power rating, it is the rate at which the bulb converts electrical energy into light and heat. A bulb with a higher wattage will use more electricity per unit of time.

If a bulb has a power rating of 50 watts, it will produce 50 joules per second. In an hour seconds it will producejoules.

When you walk a mile, your motive force is displacing your body, which is measured as the work is done. When you run the same mile, you are doing the same amount of work but in less time. The runner has a higher power rating than the walker, putting out more watts. A car with 80 horsepower can produce faster acceleration than a car with 40 horsepower. In the end, both cars are going 60 miles per hour, but the hp engine can reach that speed faster.

In the race between the tortoise and the hare, the hare had more power and accelerated faster, but the tortoise did the same work and covered the same distance in a much longer time. The tortoise showed less power. When discussing power, people are usually referring to average power, P avg. What is the power at a specific time? When the unit of time approaches zero, calculus is needed to derive an answer, but it is approximated by force times speed.

Share Flipboard Email. Andrew Zimmerman Jones. Math and Physics Expert. Andrew Zimmerman Jones is a science writer, educator, and researcher. He is the co-author of "String Theory for Dummies.In physics, power is the amount of energy transferred or converted per unit time. In the International System of Unitsthe unit of power is the wattequal to one joule per second.

In older works, power is sometimes called activity. The output power of a motor is the product of the torque that the motor generates and the angular velocity of its output shaft. The power involved in moving a ground vehicle is the product of the traction force on the wheels and the velocity of the vehicle.

The power of a jet-propelled vehicle is the product of the engine thrust and the velocity of the vehicle. The rate at which a light bulb converts electrical energy into light and heat is measured in watts — the electrical energy used per unit of time. Power is the rate with respect to time at which work is done; it is the time derivative of work:. In this case, power can be written as:. If instead the force is variable over a three-dimensional curve C, then the work is expressed in terms of the line integral:.

Hence the formula is valid for any general situation. The dimension of power is energy divided by time. Other common and traditional measures are horsepower hpcomparing to the power of a horse; one mechanical horsepower equals about As a simple example, burning one kilogram of coal releases much more energy than does detonating a kilogram of TNT[6] but because the TNT reaction releases energy much more quickly, it delivers far more power than the coal.

It is the average amount of work done or energy converted per unit of time. The average power is often simply called "power" when the context makes it clear. In the case of constant power Pthe amount of work performed during a period of duration t is given by:. In the context of energy conversion, it is more customary to use the symbol E rather than W. Power in mechanical systems is the combination of forces and movement. In particular, power is the product of a force on an object and the object's velocity, or the product of a torque on a shaft and the shaft's angular velocity.

Mechanical power is also described as the time derivative of work. In mechanicsthe work done by a force F on an object that travels along a curve C is given by the line integral :.

unit of power

If the force F is derivable from a potential conservativethen applying the gradient theorem and remembering that force is the negative of the gradient of the potential energy yields:. If a mechanical system has no losses, then the input power must equal the output power. This provides a simple formula for the mechanical advantage of the system. Let the input power to a device be a force F A acting on a point that moves with velocity v A and the output power be a force F B acts on a point that moves with velocity v B.

If there are no losses in the system, then. These relations are important because they define the maximum performance of a device in terms of velocity ratios determined by its physical dimensions. See for example gear ratios. If the component is a resistor with time-invariant voltage to current ratio, then:. The peak power is simply defined by:. If one defines the energy per pulse as:.

From Wikipedia, the free encyclopedia. Rate at which energy is transferred, used, or transformed. Derivations from other quantities.

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unit of power

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unit of power

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